# Kinetic energy of a ball

1. Nov 25, 2009

### Alu

1. The problem statement, all variables and given/known data
A 0.04-kg ball is thrown from the top of a 30-m tall building (point A) at an unknown angle above the horizontal. As shown in the figure, the ball attains a maximum height of 10 m above the top of the building before striking the ground at point B. If air resistance is negligible, what is the value of the kinetic energy of the ball at B minus the kinetic energy of the ball
at A , that is calculate (K(B) – K(A))?

2. Relevant equations

3. The attempt at a solution
I think K(B)- K(A)=K(B)=mgh is this correct?

2. Nov 25, 2009

### Feldoh

K(b)-K(a) = mgh however that doesn't tell you what K(a) is. You assumed K(a) = 0 but how can it be zero if it is moving upward initially?

3. Nov 25, 2009

### Alu

yeah right, I assumed that K(a)=0 which is not. But at least we can find K(b) which is equals to mgh where h is 30+10 m

4. Nov 25, 2009

### Feldoh

Err actually if you're just finding the difference in kinetic energy, it's just the change in potential energy so you are correct and you dont need to know K(a). I mean K(a) =/= 0 but it doesn't matter anyways.

Last edited: Nov 25, 2009
5. Nov 25, 2009

### Alu

yeah exactly, which leads to: K(b)-K(a)=mgh where h is only 30m(the difference in potential energy). which is equals to 0.04*10*30=12J